1. Field of the Invention
This invention relates to an ion trap mass analyzer, and in particular to an ion trap mass analyzer constructed from printed circuit boards.
2. Description of Related Art
Quadrupole electric fields can be generated by various kinds of quadrupole ion optics. Driven by an RF voltage, these quadrupole ion optics are used for different purposes, e.g. an ion guide for ion transmission, a quadrupole mass filter for mass analysis or ion selection, and an ion trap for storage and analysis of ions. Among these devices, a quadrupole mass filter normally needs a high vacuum to achieve its performance while the ion guide and ion trap can work at a lower vacuum. An ion trap mass spectrometer is formed by combining an ion guide and an ion trap and coupling them to an ion source. A tandem mass spectrometer, otherwise known as an MS-MS, is formed by combining two mass spectrometers in tandem with a collision cell between them. The tandem mass spectrometer provides an opportunity to elucidate the structure of complex molecules and has increasingly found new applications in analytical chemistry.
A conventional ion trap mass spectrometer is a high precision mechanical device that includes machined hyperbolic surfaces. The manufacturing and assembly of such a device is very expensive, resulting in reduced productivity, due to the high precision required in the hyperbolic surfaces. Although a high pressure ion guide does not require the same level of precision, it cannot be used as an analytical device. Since the introduction of mass-selective-ejection operational modes in ion traps, it has been realized that an ion trap electrode structure with imperfections can be employed to achieve good analytical results. For example, a stretched ion trap results in good signal intensity and higher mass resolution. Furthermore, as a lower cost solution, simpler cylindrical geometries have been considered in the construction of the ion trap instead of more complicated rotational hyperbolic geometries. This lower cost ion trap can achieve a mass resolution over 1000 by proper design of the ratios of some important dimensions, e.g. the inner diameter of the cylinder and of injection/ejection apertures, and the distance between the end plates.
One of the disadvantages of a 3D ion trap mass spectrometer is its small charge capacity. When a large number of ions (>106) are stored in the trap, the space charge inside the trap will destroy its analytical performance. Recently, it has been realized that a linear ion trap can hold at least one order of magnitude more ions than the 3D ion trap before reaching the space charge limit. Linear ion trap configurations, such as disclosed in United States Patent Application Publication No. US2003/0183759A1, filed by Jae Schwartz et al. on Feb. 3, 2003, have become increasingly popular and have already been commercialized. The structure of a conventional linear ion trap, as shown in the FIG. 1, is similar to a three segmented quadrupole mass analyzer, where the front segment and back segment are held at a DC potential higher than the DC potential of the centre segment. Ions (positive ions in this case) which satisfy the stability condition of the quadrupole are confined by a DC field along the longitudinal axis of the trap and by an RF trapping field in a direction perpendicular to the longitudinal axis. Mass analysis can thereby take place under such a trapping condition.
It is, however, difficult to fabricate a linear ion trap. The machining of segmented rods and the alignment of all rods with precise parallelism and symmetry has contributed to a high cost of manufacture.
Chinese Patent No. CN85102274 discloses a quadrupole lens structure where a square shaped boundary was made with resistive material to generate a continuous varying electric potential along the boundary. Theoretically, this approach generates an ideal quadrupole field inside the lens structure. However, in practice it is difficult to obtain the high resistive coating with high thermal stability. When a radio frequency high voltage is applied to the resistive material, the power dissipation results in an increase in temperature, which in turn varies the impedance of the device. This off-tunes the resonance of the RF generator. Although a switching circuit for driving the quadrupole field can be used to avoid the above problem, as taught in PCT Application No. PCT/GB00/03964 by Ding et al., filed Oct. 16, 2000, the uneven temperature rise can still change the field distribution inside the ion optical structure so the analytical performance is still lost.
Efforts have been made to seek new materials in the fabrication of mass spectrometers. U.S. Pat. No. 6,316,768, issued Nov. 13, 2001 to Alan Rockwood et al., discloses a Time-of-Flight (ToF) mass spectrometer constructed with printed circuit board (PCB) materials. In the disclosure, it also mentions creating RF multipole ion guides utilizing PCBs whereby traces on the PCBs are disposed such that there exists an axial voltage gradient. However, the disclosed PCB structure can not be used as an ion trap because it does not provide a means to stop ions from escaping the structure along its axis and because the amount of octopole field induced by the shape of traces prevents the formation of a trapping field that is suitable to mass analysis.
It is an object of the present invention to provide techniques to produce low cost ion trap mass analyzers with good mass resolution. It is yet another object of the present invention to integrate optics for ion introduction and for mass analysis so that the construction and assembling of the mass spectrometer can be simplified while still being precise. It is a further object of the present invention to provide a high sensitivity, high throughput ion trap mass analyzer by combining ion traps in parallel while keeping the manufacturing cost low.